" Since the fall of Kiev in 1240, the western lands of Galicia and Volhynia had served as the stage for major developments in Ukrainian history. However, by the end of the i6th century, the focus of events shifted back to the east, to the lands of the Dnieper basin that had long been partially depopulated. In that vast frontier, which at that time was specifically referred to as Ukraina - the land on the periphery of the civilized world - the age-old struggle of the sedentary population against the nomads flared up with renewed intensity, fueled by the bitter confrontation between Christianity and Islam. The oppressive conditions that obtained in the settled western areas provided numerous recruits who preferred the dangers of frontier life to serfdom. As a result, a new class of Cossack-frontiersmen emerged. Initially, the Cossacks concentrated on pushing back the Tatars, thereby opening up the frontier to colonization.

But as they honed their military and organizational skills and won ever more impressive victories against the Tatars and their Ottoman Turkish over lords, Ukrainian society came to perceive the Cossacks not only as champions against the Muslim threat, but also as defenders against the religion, national and socioeconomic oppression of the Polish szlachta. Gradually, moving to the forefront of Ukrainian society, the Cossacks became heavily involved in the resolution of these central issues in Ukrainian life and, for the next several centuries, provided Ukrainian society with the leadership it had lost as a result of the Polonization of the Ukrainian nobility. "

― Orest Subtelny , Ukraine: A History